The neurobiology of vision has emerged as the premier model for understanding higher cognitive processes such as decision-making. Indeed much is now understood about the neural mechanisms that underlie both the speed and accuracy of simple decisions between two alternative interpretations of an ambiguous visual stimulus. In a motion discrimination task, for example, neurons in the parietal cortex of the monkey accumulate evidence for the alternatives by integrating, as a function of time, the responses of neurons in the visual cortex. The decision process is thought to terminate when the accumulation of evidence reaches a criterion level. Although these neurons lend insight into the brain's computations, it is not known whether they actually cause the brain to commit to a decision. Nor is it known how they govern more complex decisions. The proposed research tests the hypothesis that neurons in parietal cortex play a causal role in decisions, and it examines the role of these neurons in more complicated decisions that are more like the ones humans rely on to perform cognitive tasks. The experiments combine neural recording and stimulation in rhesus monkeys with behavioral measurements in both monkeys and humans. Three specific aims are planned. 1) Microstimulation of the lateral intraparietal area (LIP) will elucidate how changes in the activity of LIP neurons affect perceptual decisions about an ambiguous motion stimulus and whether these neurons cause the brain to commit to a decision. 2) Neural recordings in area LIP along with behavioral testing in monkey and man will reveal the mechanism underlying perceptual decisions when there are more than two alternatives to choose among. 3) Neural recordings in area LIP along with behavioral testing in monkey and man will reveal how the brain combines information from several sources to reach a decision in a probabilistic classification task. Understanding the neural mechanisms that underlie decisions will help to elucidate the principles of neuroscience that give rise to cognition and its disorders. The proposed experiments leverage a solid foundation of knowledge of visual neuroscience toward an understanding of cognitive reasoning. Such understanding promises to provide the means to promote recovery of both sensory and intellectual function in the face of neurological disease.